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      <title>15、CountDownLatch并发编程中的同步利器 - 交互式测验</title>
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              <h1>15、CountDownLatch并发编程中的同步利器 - 交互式测验</h1>
              <p>检验你对文档内容的理解程度</p>
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  {
    "question": "根据文档，`CountDownLatch`的核心工作机制是什么？",
    "options": [
      "它维护一个内部计数器，允许一个或多个线程等待，直到计数器归零时才被唤醒继续执行。",
      "它为每个线程提供一个许可证，只有获得许可证的线程才能执行，用于控制并发访问量。",
      "它创建一个屏障点，所有线程必须在该点互相等待，直到所有线程都到达后才能一起继续执行。",
      "它必须等待一个指定的线程执行完毕后，其他线程才能开始执行。"
    ],
    "correctAnswerIndex": 0,
    "explanation": "CountDownLatch 的核心思想是维护一个计数器。当一个或多个线程调用 await() 方法时，它们会阻塞等待。其他线程完成其任务后调用 countDown() 方法使计数器减一。当计数器值减为零时，所有因调用 await() 而等待的线程都会被释放，得以继续执行。",
    "source": "文档中「一、CountDownLatch 基础概念」->「1.1 什么是 CountDownLatch？」部分",
    "hint": "文档中提到了一个比喻：'就像一个倒计时器，只有当计数归零时，等待的线程才能继续执行。'"
  },
  {
    "question": "文档中强调`CountDownLatch`有一个重要的特性，这导致了它与`CyclicBarrier`的一个关键区别。这个特性是什么？",
    "options": [
      "`CountDownLatch`的性能在高并发下会急剧下降。",
      "它的计数器一旦归零就无法重置，因此是“一次性”的同步工具。",
      "`CountDownLatch`只能用于主线程等待子线程的场景。",
      "它的 `await()` 方法不支持设置超时时间。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "文档明确指出，CountDownLatch 的计数器一旦归零就不能重新设置或重用。如果需要在一个循环或多阶段任务中重复使用同步功能，应考虑使用 CyclicBarrier，因为 CyclicBarrier 是可重置和可重用的。",
    "source": "文档中「4.3 注意事项与最佳实践」和「5.1 CountDownLatch vs CyclicBarrier」部分",
    "hint": "思考一下，如果一个任务需要分多个阶段，并且每个阶段都需要类似的同步，`CountDownLatch` 是否能满足需求？文档在“注意事项”部分提到了这一点。"
  },
  {
    "question": "在“并发测试”的应用场景中，文档使用了`startLatch`和`endLatch`两个`CountDownLatch`实例。其中，`startLatch`的主要作用是什么？",
    "options": [
      "确保所有测试线程能够同时开始执行，以模拟真实的并发请求。",
      "用于在所有请求完成后，统计测试的总耗时。",
      "在测试开始前，用于初始化线程池资源。",
      "记录测试中失败的请求数量。"
    ],
    "correctAnswerIndex": 0,
    "explanation": "在并发测试场景中，`startLatch` (通常初始化为1) 的作用是作为一个“发令枪”。所有请求线程在准备好后都调用 `startLatch.await()` 等待。当主线程调用 `startLatch.countDown()` 后，所有等待的线程会同时被唤醒，从而实现最大程度的并发。",
    "source": "文档中「2.3 应用场景三：并发测试」部分",
    "hint": "文档中描述 `startLatch` 的用途是：'用于控制所有线程同时开始，模拟真实的并发请求场景'。"
  },
  {
    "question": "根据文档中的源码解析，`CountDownLatch`的内部实现是基于哪个核心的Java并发框架？",
    "options": [
      "Java内存模型 (JMM)",
      "Fork/Join 框架",
      "AQS (AbstractQueuedSynchronizer)",
      "NIO (Non-blocking I/O)"
    ],
    "correctAnswerIndex": 2,
    "explanation": "文档的“源码解析”章节明确指出，CountDownLatch 的实现基于 AQS（AbstractQueuedSynchronizer）框架。它内部有一个继承自 AQS 的 Sync 类，并利用 AQS 的 state 变量作为计数器，通过重写 `tryAcquireShared` 和 `tryReleaseShared` 方法来实现线程的等待和唤醒。",
    "source": "文档中「三、源码解析」部分",
    "hint": "文档提到 'CountDownLatch 的实现基于 AQS...这也是 Java 并发包中许多同步器的基础。'"
  },
  {
    "question": "为了防止因工作线程抛出异常而导致`countDown()`方法未被调用，从而使主线程永久等待，文档推荐的最佳实践是什么？",
    "options": [
      "在工作线程的 `catch` 块中调用 `countDown()`。",
      "在工作线程的 `finally` 块中调用 `countDown()`。",
      "为 `await()` 方法设置一个非常长的超时时间。",
      "在启动线程前就调用一次 `countDown()` 作为预减。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "为确保无论工作线程是否成功完成或抛出异常，计数器都能被正确地递减，`countDown()` 方法应该被放置在 `finally` 块中。`finally` 块中的代码保证了即使在 `try` 块中发生异常，它也一定会被执行。",
    "source": "文档中「4.3 注意事项与最佳实践」部分",
    "hint": "在Java的异常处理机制中，哪个代码块可以保证无论是否发生异常都会被执行？"
  },
  {
    "question": "`CountDownLatch`的 `await(long timeout, TimeUnit unit)` 方法与不带参数的 `await()` 方法有何不同？",
    "options": [
      "前者性能更高，但精度较低。",
      "前者会阻塞当前线程，直到计数器归零或等待超过指定时间，并返回一个布尔值表示是否在超时前完成。",
      "前者只在指定的 `TimeUnit` 内有效，例如只在秒级有效。",
      "前者如果超时会直接抛出 `TimeoutException` 异常，而后者会无限期等待。"
    ],
    "correctAnswerIndex": 1,
    "explanation": "带超时的 `await` 方法为等待操作增加了一个时间上限。如果在指定时间内计数器归零，它会返回 `true`。如果超过了指定时间计数器仍未归零，它会返回 `false` 并停止等待，允许线程继续执行。这可以有效避免因任务长时间未完成而导致的死锁或无限等待。",
    "source": "文档中「4.1 带超时的等待」部分",
    "hint": "思考一下，如果一个任务可能永远无法完成，我们应该如何避免让等待它的线程陷入永久阻塞？"
  },
  {
    "question": "将`CountDownLatch`与`Thread.join()`进行比较，以下哪个说法是正确的？",
    "options": [
      "`join()` 必须等待整个线程执行完毕，而`CountDownLatch`的 `countDown()` 可以在线程执行过程中的任何时刻被调用，提供了更细粒度的控制。",
      "`join()` 比 `CountDownLatch` 更适合协调多个并行任务的结果汇总。",
      "只有 `join()` 支持中断操作，`CountDownLatch`的`await()`会忽略中断。",
      "`CountDownLatch` 只能等待一个线程，而`join()`可以等待多个线程。"
    ],
    "correctAnswerIndex": 0,
    "explanation": "`Thread.join()` 的作用是等待一个线程的生命周期结束。而`CountDownLatch`的等待条件是计数器归零，与线程是否结束无关。一个任务（线程）可以在其执行过程中的某个关键节点（而非必须是结束时）调用`countDown()`，这为复杂的同步场景提供了更高的灵活性和更细的控制粒度。",
    "source": "文档中「5.3 CountDownLatch vs Join」部分",
    "hint": "文档对比了两者的灵活性和粒度，指出其中一个'不必等待线程结束'。"
  }
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                '<p>' + correctCount + ' / ' + totalQuestions + '</p>' +
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                '<div class="score-card">' +
                '<h3>正确率</h3>' +
                '<p>' + accuracy + '%</p>' +
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                '<div class="score-card">' +
                '<h3>错误率</h3>' +
                '<p>' + errorRate + '%</p>' +
                '</div>' +
                '<div class="score-card score-details">' +
                '<table>' +
                '<tr><td>正确</td><td>' + correctCount + '</td></tr>' +
                '<tr><td>错误</td><td>' + wrongCount + '</td></tr>' +
                '<tr><td>未答</td><td>' + skippedCount + '</td></tr>' +
                '</table>' +
                '</div>' +
                '</div>' +
                '<div class="report-actions">' +
                '<button id="review-btn">回顾测验</button>' +
                '<button class="primary" onclick="location.reload()">再试一次</button>' +
                '</div>' +
                '</div>';

            let reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }
            quizContentEl.insertAdjacentHTML('beforeend', finalScoreHtml);
            document.getElementById('review-btn').addEventListener('click', reviewQuiz);
        }

        function reviewQuiz() {
            const reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }

            document.querySelector('.quiz-body').style.display = 'block';
            document.getElementById('quiz-footer').style.display = 'flex';
            
            currentQuestionIndex = 0;
            loadQuestion();
            updateNavigationButtons(); // Ensure nav buttons are correctly displayed
        }

        // Event Delegation for options
        optionsListEl.addEventListener('click', (event) => {
            if (event.target && event.target.matches('li.option-item')) {
                // Check if the question has already been answered
                if (userAnswers[currentQuestionIndex] === null) {
                    const selectedIndex = parseInt(event.target.dataset.index, 10);
                    selectOption(selectedIndex);
                }
            }
        });

        prevBtn.addEventListener('click', goToPreviousQuestion);
        nextBtn.addEventListener('click', goToNextQuestion);
        hintBtn.addEventListener('click', showHint);
        
        // 初始加载
        loadQuestion();
      </script>
    </body>
    </html>
  